During heart development one of the most significant differentiation events by non-myocardial cells is the induction of endocardial cells to form the progenitors of the valves. Although the majority of the endocardium in the atrioventricular canal become activated, only a subset of cells will display loss of cell-cell contact, and will undergo an epithelial-mesenchymal cell transformation.. One element of valve formation that is not well characterized is the process of cell-cell separation that signals the start of this transformation. The present study is focused upon a few key elements of this process and the experimental design is based upon the collaborating expertise of two laboratories. Cadherins, a family of integral membrane cell-cell adhesion receptors, play an important role in controlling cellular behavior, such as growth, migration, and differentiation. The cadherins form a complex with cytoplasmic proteins, the catenins, which regulate cell adhesion in response to signaling pathways. Experiments are proposed to study the hypothesis that the endocardial specific adhesion molecule, cadherin-5 (VE-cadherin), is a critical component of this process. Additional components of endocardial cell-cell adhesion, such as N-cadherin, and beta-catenin, will be compared. While cadherins are the adhesive components of the intercellular junction, beta-catenin functions not only in cell adhesion but in signal transduction by association with DNA- binding proteins. We propose that signaling pathways regulating both cadherin phenotype and cadherin/catenin cell adhesion, as well as, beta- catenin nuclear signaling are necessary for cell transformation. The specific aims are: 1) To characterize the spatial and temporal expression pattern of molecules that play a role in cell-cell adhesion during the period of valve formation in the heart. 2) To examine the functional role of the cadherin-5 during transformation of AV Canal endocardium into cushion mesenchyme. 3) To examine the regulation of beta-catenin with its binding cell-cell adhesion versus cell signaling during cell transformation. 4) To determine the role of beta catenin in proposed study is a tissue culture system that enables examination of a pure population of cushion mesenchyme separated from endocardium. These studies will provide new information relevant to normal development of the vertebrate heart and congenital heart defects, separated from endocardium.